Many chemical carcinogens have been found to induce point mutations. Recent developments, however, suggest that the induction of point mutations may not be the most important action of carcinogens, and the ability to cause gene amplification and rearrangements might correlate better with carcinogenicity. Since carcinogenesis is a multi-step phenomenon, different genetic events may be involved in different stages. This proposal seeks to develop a system allowing the study of a variety of genetic end-points in the same cell system. We will utilize Chinese hamster V79 cells, a line already in extensive use in mutagenesis assays, to construct a line which carries one copy of the herpes simplex virus thymidine kinase (HSV-tk) gene. A series of TK-mutants, revertible by specific mechanisms (frameshifts, various types of base-pair substitutions) will then be isolated from the transformed (HSV-tk) line. This system will set the stage for a powerful approach to studying the molecular events caused by carcinogens. Because the HSV-tk gene is easily identifiable and recoverable, experiments to determine the exact sequence of altered genes can eventually be performed. This system should also serve as a useful screening system whereby the types (if any) of gene mutations, gene amplification and rearrangements can be assayed in the same cells treated with the same chemical agents. It may then be possible to correlate a particular stage of carcinogenesis with a particular spectrum of genetic effects.